1. Field of the Invention
The present invention relates to a sheet processing apparatus that performs post processing on a bent portion of a folded sheet bundle formed by folding sheets stacked one upon another, and more particularly to a sheet processing apparatus that flattens a folded spine portion of a saddle-stitched sheet bundle to thereby improve the appearance of the sheet bundle, and an image forming apparatus including the sheet processing apparatus.
2. Description of the Related Art
Some image forming apparatuses are provided with a sheet processing apparatus for performing folding processing and the like on sheets having images formed thereon by an image forming section. Further, some sheet processing apparatuses of this type perform processing for folding a sheet bundle and then flattening a curved folded spine, which is a bent portion of the folded sheet bundle, so as to improve the quality of the folded spine and stackability of the sheet bundle (see e.g. Japanese Patent Laid-Open Publication No. 2006-36493).
In the conventional sheet processing apparatus, flattening of the folded spine is performed by the following steps: In a first step, the folded sheet bundle is conveyed by conveying rollers, and the curved folded spine of the sheet bundle is brought into abutment with a positioning member, for positioning of the sheet bundle. In a second step, a portion of the sheet bundle close to the folded spine is clamped and fixed by a holding member, and then the positioning member is retracted. In a third step, the folded spine is flattened by causing a pressing roller to travel along the curved folded spine of the sheet bundle while urging the pressing roller against the curved folded spine. In this flattening step, by changing the position of the positioning member in a conveying direction of the sheet bundle, it is possible to adjust the amount of crushing of the folded spine into a flat state.
In the conventional sheet processing apparatus, however, the positioning member, a mechanism for retracting the positioning member, and a mechanism for moving the positioning member in the conveying direction are required, which increases the size and costs of the sheet processing apparatus.
As a solution to the inconveniences, it is envisaged to configure the sheet processing apparatus such that the sheet bundle is stopped by stopping the driving of the conveying rollers, and thereby omit the positioning member and the mechanisms for retracting and moving the positioning member to reduce the number of component parts and simplify the structure of the sheet processing apparatus, and thereby reduce the size and costs of the sheet processing apparatus.
However, in a case where the sheet processing apparatus is configured such that the sheet bundle is stopped by stopping the driving of the conveying rollers, as described above, an error in the stop position of the sheet bundle increases due to variation in a braking distance over which the sheet bundle is moved when a motor for driving the conveying rollers is stopped and a play of a driving mechanism. Therefore, when the sheet processing apparatus is configured as described above, the amount of crushing of the folded spine of the sheet bundle is unstable to cause variation in finish quality.